Ca2+ mobilizing hormones such as norepinephrine acting on Alpha-1 adrenergic receptors are thought to initiate receptor coupling events by their ability to activate a plasma membrane phospholipase C which selectively hydrolyzes plasma membrane polyphosphoinositides. In arterial smooth muscle both intracellular Ca2+ release and Ca2+ influx result from Alpha-1 receptor activation, however, the sequences of events causing these Ca2+ movements has not been established. Our studies during years one and two of this project show that PI cycle phospholipids are selectively hydrolyzed, indicating that the products of phosphatidyl inositol 4.5 bisphosphate hydrolysis (DAG and IP3) may be responsible for Ca2+ influx and Ca2+ release respectively. In the current proposal this hypothesis is to be tested and the mechanism by which these two agents cause Ca2+ movements in arterial smooth muscle is to be investigated. Conditions which modify Alpha-1 receptor coupling events will also be studied. Three areas can be identified: 1. Verification of IP3 and DAG formation and delineation of their relationship to Ca2+ release and Ca2+ influx. This includes a study of the action of phorbol esters which activate protein kinase C in a manner similar to DAG and a study of the possible role of Na+/H+ exchange in Ca2+ influx. 2. Dynamic aspects of Alpha-1 receptor function. Consequences of pre-activation of the PI-cycle pathway by either Alpha-1 agonists or angiotensin II will be determined along with the influence of c-AMP- or c-GMP- dependent protein kinase systems on Alpha--1-receptor coupling events. 3. Impact of Alpha-1 receptor-induced non-PI cycle phospholipid changes. Experiments will detail receptor-induced changes in phosphatidylcholine synthesis to determine how they may modulate Alpha-1 receptor coupling events. Particular emphasis will be placed on observed species differences between rat and rabbit arteries. This project should not only provide specific information on the mechanism of Alpha-1-adrenergic coupling events in arteries, but will help to identify conditions which may modulate performance of this receptor system under physiologic conditions. Our results will have a direct bearing on our knowledge of cardiovascular physiology and pathophysiology and may provide insight into better management of hypertension and related disorders.